This disclosure relates generally to the field of well log data. More particularly, the disclosure relates to methods for determining attributes of well log data to assist in determining characteristics of subsurface formations.
Well logs are measurements of physical parameters of the subsurface formations that are penetrated by a wellbore, with respect to depth (axial position along the wellbore, which may or may not be converted into “true vertical depth” or linear distance from the surface to a particular position in the wellbore). The measurements may include, for example and without limitation, acoustic data, resistivity data, radiation source activated nuclear data, temperature data, and naturally occurring radioactivity data. Data from well logging is typically displayed in a graphical form with the measured parameter displayed on the horizontal axis, and the depth at which the measurement was made on the vertical axis, as shown in FIG. 1. These data are used to make inferences about the rock and fluid properties of subsurface formations, such as rock mineral composition, clay volume, porosity, permeability, and fluid saturation, to correlate geological formations and properties between wellbores, and to make interpretations of depositional environment and lithologic facies.
Interpretation of the depositional environment and lithological facies is typically performed by making qualitative descriptions of certain measured parameters, and by describing the general shapes of the well log data when plotted with respect to depth. For example, a well log such as the one shown in FIG. 2 is often described as having a “coarsening upwards” pattern, inasmuch as the measurement of the formation's natural radioactivity (corresponding to clay mineral content) decreases with decreasing depth. Furthermore, the formation depicted in FIG. 2 might be described as “clean” (i.e., substantially free of clay minerals) at the uppermost levels where the values for measured radioactivity are low and as “shaly” where the measured values are higher. These types of descriptions are qualitative and are performed manually by a skilled interpreter using prior knowledge of the art of geological and petrophysical interpretation.
The well log data observed by an interpreter are typically classified by the well log curve shape. Examples of some commonly classified log shape descriptions are shown in FIG. 3 at 90 through 97. These types of interpretations are typically performed manually, as stated above, and are based on the interpreter's subjective opinion. There are numerous shortcomings associated such interpretive techniques, including: difficulties associated with consistent interpretations among interpreters; the inability to rapidly make interpretations using computer algorithms; the inability to quantify the differences between well logs within a classification or among various classifications; and the requirement that the interpreter be highly skilled in the art of well log interpretation.
There exists a need for more reliable, consistent methods for characterizing formation characteristics based on changes in well log data values with respect to depth in a wellbore.